A powder compacting technique is used to manufacture various types of parts to be made of a ceramic or a metal. For example, a sintered body of a ceramic or a metal is manufactured by sintering a compact that has been obtained in a predetermined shape by subjecting a powder material to a powder compaction process. By subjecting the sintered bodies to a finishing process thereafter to adjust the sizes and shapes thereof, final products are obtained.
Generally speaking, the quality of a sintered body (in terms of physical properties or configuration) is determined by the quality of a compact. Also, the compactability depends on the particle size distribution or the particle shapes of the powder material. For these reasons, various powder compacting methods have been proposed so far to obtain a compact of quality.
For example, a sintered magnet of a rare earth alloy may be produced by performing the process steps of:                (1) melting a material metal at a high temperature to obtain an ingot of a rare earth alloy with a predetermined composition;        (2) pulverizing this alloy ingot to obtain a rare earth alloy powder of a small size;        (3) compacting the resultant alloy powder (to the surface of which a lubricant is added depending on the necessity) under a magnetic field to obtain a compact in a predetermined shape;        (4) sintering this compact at a high temperature (e.g., about 1,000° C. or more) to obtain a sintered magnet;        (5) further subjecting the resultant sintered magnet to a heat treatment called an “aging treatment” to improve the magnetic properties thereof; and        (6) grinding the surface of the sintered magnet to adjust the sizes and shapes thereof.        
In compacting an alloy (or magnet) powder material for use to produce the magnet described above, the orientation directions of the alloy particles need to be aligned with a predetermined direction under a magnetic field. It is known that a rare earth sintered magnet with excellent magnetic properties can be obtained by using an alloy powder that has been prepared by a strip casting process. However, it is particularly difficult to obtain a compact of quality from this alloy powder. This is because the alloy powder obtained by a strip casting process or any other rapid cooling process has a small mean particle size (e.g., about 2 μm to about 5 μm), a narrow particle size distribution and a low flowability (or compactability). It should be noted that the “mean particle size” means herein a mass median diameter unless stated otherwise.
To produce a rare earth sintered magnet with excellent magnetic properties, the present inventors tested various conventional powder compacting methods to discover that those methods had the following problem. This problem will be described with reference to FIGS. 13(b) and 13(c). It should be noted that the feature of an inventive powder compacting method as shown in FIG. 13(a) will be described later.
Suppose an alloy powder material, prepared by a strip casting process, is loaded into a cavity and pressed by an upper punch and a lower punch (typically made of a metal (e.g., SUS 304)) in accordance with a normal uniaxial compacting method (typically a die-pressing method). In that case, if the alloy powder material 10 has a fill density (or loading weight) distribution as shown in FIG. 13(b) (where H indicates a high density and L indicates a low density), then the resultant compact 20 should also have a non-uniform density distribution corresponding to the fill density distribution. Also, even if the cavity is filled with the alloy powder material at a sufficiently uniform density, the alloy powder material may still show some variation in its fill density when subjected to a magnetic field alignment process during the pressing process. Such a variation is caused by the field strength (or flux density) distribution during the magnetic field alignment process. A higher pressure is normally applied to a portion with a higher fill density. Accordingly, when the alloy powder material is subjected to the pressing process, such a variation in its density is amplified. And if such a density variation is significant, then the compact may crack, chip or deform as a result.
Furthermore, when the compact 20 with such a non-uniform density distribution is sintered, the resultant sintered body 30 should be further deformed. This is because there is a correlation between the rate of shrinkage of the compact 20 through the sintering process and the density of the compact 20. That is to say, the shrinkage rate changes with the density distribution. This problem is particularly noticeable in a compact with a low density. Also, a thin compact is considerably affected by the distribution in the shrinkage rate, easily cracks or chips, and is likely deformed significantly.
On the other hand, it is known that a quality compact of a magnetic powder material can be obtained by a rubber pressing method. In this method, a magnetic powder material is loaded into a mold made of rubber and then immersed in a liquid medium such that a hydrostatic pressure is applied to the magnetic powder material by way of the rubber mold. According to this rubber pressing method, a pressure can be applied isotropically to the magnetic powder material. Thus, even if the magnetic powder material that has been loaded into the mold has a non-uniform density distribution, a compact with a uniform density distribution can still be obtained. Unfortunately, though, the rubber pressing method is a sort of hydrostatic pressure pressing process with very low productivity and is hard to apply industrially.
Thus, to increase the low productivity of the rubber pressing process, Japanese Patent Gazette for Opposition No. 55-26601 proposes a parallel die-pressing method in which a pre-molded rubber container is put into a die, filled with an alloy powder, and then pressure is applied thereto in the same direction as the magnetic field. In the pressing method disclosed in Japanese Patent Gazette for Opposition No. 55-26601, however, if a powder material with a low fill density, which has been loaded by a natural loading technique, for example, is pressed, then the resultant compact likely cracks, chips or deforms.
To overcome such a problem, Japanese Laid-Open Publication No. 4-363010 proposes a method of die-pressing a magnetic powder material that has been loaded into a mold, having at least a rubber side surface and a bottom, at a high density (which is at least 1.2 times as high as the natural fill density). According to this method, however, the magnetic powder material 10 likely has a non-uniform fill density distribution while being loaded into such a rubber mold at a high density. Thus, the resultant compact 20 can have a uniform compact density as shown in FIG. 13(c). But since the outer shape of the compact 20 reflects its fill density distribution, it is difficult to obtain a compact in a predetermined shape. For that reason, to process a sintered body 30, obtained from such a compact 20, into the predetermined shape, all of the surfaces of the sintered body 30 must be machined. Also, this method requires high-density filling. Accordingly, when a magnetic powder with a small mean particle size and a narrow particle size distribution (e.g., a rare earth alloy powder obtained by a strip casting process) is used, the powder easily sticks together, thus causing a significant variation in fill density. As a result, the problem becomes even more noticeable.
As described above, none of the conventional techniques can compact a powder material with a non-uniform fill density distribution at a high productivity with cracking, chipping or deformation of the compact minimized. In particular, none of those techniques can compact a powder material with a low fill density (e.g., the rare earth alloy powder material described above) at a high productivity.
In order to overcome the problems described above, an object of the present invention is to provide a powder compacting method and apparatus that can make a compact with a uniform density distribution at a high productivity even from a powder material with a non-uniform fill density distribution, and a method for producing a magnet by using them.